1. Field of the Invention
This invention relates generally to microelectromechanical devices used in electronic circuits and is directed in particular to a nonvolatile memory cell using an integrated microelectromechanical device made by a photolithographic process.
2. Description of Related Art
Conventional nonvolatile VLSI memory technologies use a dual polysilicon process consisting of a stacked control gate and a floating gate over a MOSFET (metal oxide semiconductor field effect transistor) channel. Under the proper voltage conditions, the floating gate becomes charged with carriers that have tunneled through the oxide from the MOSFET channel. This charge alters the device threshold enough to be electrically sensed. The level of the charge corresponds to either a 0 or 1 signal so that the device can be used as a memory cell in, for example, random access memory in a computer.
Dual polysilicon device designs can suffer from the need for extra high voltage power supplies and from gate oxide stress degradation. Tunneling currents are usually low, so write/erase times, even though infrequent, are long compared to other solid state memory technologies.
Accordingly, there has been a need in the prior art for a nonvolatile memory cell which is not susceptible to gate oxide degradation. Furthermore, it would be advantageous for such a cell to be able to use a single power supply and be designed to operate at low voltages. Such a memory cell should have faster write times, support write/verify functions and have retention times comparable to conventional NVRAM cells.
Halg, in the article "On a Nonvolatile Memory Cell Based on Micro-Electro-Mechanics" IEEE PROC. OF MICROELECTROMECHANICAL SYSTEMS, 1990, p. 172, discloses a surface micro-machined bridge which is logitudinally stressed so that it buckles and becomes mechanically bistable, and performs the memory function. Read and write operations are performed by sensing the capacitance of the bridge and by electrostatically switching the bridge from one stable state into the other.
Other micro-mechanical devices have been suggested for various uses but not been disclosed or suggested for use as nonvolatile memory cells. For example, U.S. Pat. No. 5,638,946 discloses a micro-mechanical switch which may be utilized in logic gates such as NAND gates, NOR gates and inverters and/or as a memory element. However, the switch utilizes an insulated cantilever beam and provides no storage means within the switch device.
U.S. Pat. No. 5,472,539 also discloses a micro-mechanical switch with a pull-down electrode and a cantilever beam, but makes no disclosure or suggestion of such a switch as a memory cell.
U.S. Pat. No. 5,262,000 discloses a method of fabricating a microelectromechanical switch with a beam of polysilicon with a pull-down electrode but discloses no memory cell which may be made from such a device.
U.S. Pat. No. 5,103,279 discloses a microelectromechanical sensor having a field effect transistor formed in a portion of a substrate which may be used as a pressure and acceleration sensor. Again, no use of a memory cell is disclosed or suggested. Other microelectromechanical switches are disclosed in U.S. Pat. Nos. 5,637,904, 5,536,963, 5,454,904, 4,979,149, 4,356,730, 5,677,823, and 5,538,753.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved nonvolatile memory cell which is not susceptible to gate oxide degradation.
It is another object of the present invention to provide a nonvolatile memory cell which may use a single power supply and can operate at low voltages if desired.
It is yet another object of the present invention to provide a nonvolatile memory cell which has faster write times than conventional nonvolatile memory cells.
It is a further object of the present invention to provide a nonvolatile memory cell which has retention times comparable to conventional nonvolatile memory cells.
It is yet another object of the present invention to provide a nonvolatile memory cell which can support write/verify functions.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.